Executive failure hypothesis explains the trait-level association between motivation and mind wandering

Mind wandering (MW) is commonly observable in daily life. Early studies established an association between motivation and MW at the trait level using a questionnaire survey. Considering that the mechanism of state-level association between them is known, this study was conducted to replicate the trait-level association and determine its possible mechanisms. Two independent samples were analysed using several questionnaires, which included motivation and MW. General one- and multi-dimensional scales were administered for both variables. Besides the successful replication of the significant association between motivation and MW at the trait level, we found that people with low levels of executive function experience high rates of spontaneous MW. This finding indicates that the underlying mechanism of trait-level association is the executive failure hypothesis, which postulates that a failure of executive control during task-related objectives evokes MW. Further, the motivation–MW relationship exhibits a different psychological basis at the state and trait levels.

www.nature.com/scientificreports/ task, individuals with greater cognitive ability could let their mind wander to task-irrelevant information in an adaptive manner that may cause a positive association. In trait level, accordingly, executive function could exhibit both ways of association with MW because there are tons of tasks with various degrees of difficulties in daily life, which is still unknown. It is reasonable to inspect the motivation-MW association via an index of executive function. Simultaneously, a deterioration in executive function can cause a clinical level of amotivation [16][17][18] , whose association can be observed in healthy individuals 19,20 . This study views motivation as a continuum to the clinical state of apathy. In clinical populations, people who are consistently unmotivated are deemed apathetic, defined as the lack of motivation that is not attributable to a diminished level of consciousness, cognitive impairment or emotional distress 16,17 . As per the definition, this concept could be applied to healthy individuals 19,20 , especially when a comprehensive, simplified scale to assess general motivation in daily life is lacking. This is because traditionally, motivation has been understood as a state that is assessed in many fields, including psychology, during specific situations using explicit goals, such as provocation using various forms of incentives (for a review, see Braver et al. 21 ). Previous clinical studies have indicated that apathy is multi-dimensional and can be grouped as executive, emotional and initiation-related types 17,18 . For example, the executive type of apathy refers to amotivation for planning, organisation or attention, which is apparently owing to the deterioration of executive function. Thus, we utilised the dimensional apathy scale (DAS), which comprises the executive (dasEx), emotional (dasEm) and behavioural/cognitive initiation (dasIni) sub-factors of apathy 18 . This study aims to investigate whether the executive type of amotivation is specifically associated with MW. Hereafter, we use the term (a)motivation instead of apathy in order to not focus on the participants' clinical aspects.
In summary, the current study aims to uncover the psychological mechanism underlying the trait-level association by considering the intentionality of MW and executive function. If the trait-level association between motivation and MW depends on the same mechanism for the state level, intentionality should affect the relationship wherein a deliberate type of MW has a stronger relationship with motivation than spontaneous MW. Alternatively, if the executive control is the key, the measurements related to executive function would mediate the motivation-MW relationship. To these ends, Study 1 first intends to determine whether replicating the significant association between motivation and MW at the trait level is confirmed by using an online survey. Further, this study adopts several mediation models besides simple correlation analyses wherein each component (i.e. spontaneous/deliberate components of MW, executive component of motivation and both) mediates the association between motivation and MW in general. Notably, although mediation analysis is used in this study, we could not assume the causality among the variables. We could only expect that the covariance between motivation and executive function could cause MW. In Study 2, another independent sample was used with the objective of conducting self-replication within the study to enhance the reliability of the findings in Study 1 (in which the executive function was assessed using a specialised measurement), that is, the effortful control scale (ECS) 22,23 was used to promote the concept that executive function mediates the relationship between motivation and MW. Specifically, this study conducted mediation analyses to verify whether executive control (i.e. ECS) mediates the effect of motivation on MW.

Study 1
Participants. This study used an online survey. All participants were recruited via iBRIDGE Corporation, a data collection company, and are assumed to be Japanese based on their location. They were compensated by a small amount of 'points' set by the data collection company. One thousand participants (aged 20-69 years) were recruited with the objective of including 200 participants in every age group (i.e. 20-29, 30-39, and so on), which resulted in 1009 participants in total. Informed consent was obtained from all participants. The appropriate ethics committee approved the study, which was conducted in accordance with the Declaration of Helsinki (1975, as revised in 2008).
Three lure items (i.e. 'Please select the most correct choice for this item') were added to identify participants who answered without paying sufficient attention to the items. The study was concerned about a possible satisficing behaviour from the participants, which may contaminate the data 24 . Participants who failed to follow the instructions were excluded from the analyses. Finally, data from 587 participants were analysed (mean age = 47.3 ± 13.5 years; women = 301). The sample size met the rule of thumb for regression analysis, that is, 100 participants + 100 per predictor variable 25,26 . Measurements. This study investigated the level of motivation (or apathy) and MW at the trait level. Two questionnaires were used to assess motivation and MW, namely, a one-dimensional scale and a multi-dimensional one.
Scales for motivation. Apathy scale (AS). AS is a 14-item questionnaire that asks participants if they have experienced a lack of motivation 27 . Each item is scored from 0 to 3; the total scores range from 0 to 42, with higher scores indicating increased amotivation. AS is used worldwide, and it exhibits good validity and reliability. This study employed the Japanese version of the scale 28 . Sample items include the following: ' Are you interested in learning new things?' and 'Do you have the energy for daily activities?' Dimensional apathy scale (DAS). DAS is used to assess apathy in a multi-dimensional manner 18 , using three dimensions, namely, executive, emotional and initiation, which are based on rigorous clinical and neurological observations 17 . Each dimension is assessed using eight items, with scores ranging from 0 to 3. In total, the subscores ranged from 0 to 24 for each dimension, with higher scores indicating higher levels of amotivation, which is similar to what holds for AS. We utilised the Japanese version of the scale 29 . Sample items for the executive, Scales for mind wandering. Mind wandering questionnaire (MWQ). MWQ is a single-factor questionnaire that assesses an individual's tendency towards MW 30 . MWQ comprises five items rated using a 6-point Likerttype scale. The total scores ranged from 5 to 50, with higher scores indicating higher levels of MW tendency. This study used the Japanese version of the scale 31 . A sample item is as follows: 'I have difficulty maintaining focus on simple or repetitive work' .
Mind wandering: deliberate (MW-D) and spontaneous (MW-S). MW-D and MW-S were used to assess MW with and without intention, respectively 32 . Each scale comprises four items rated using a 7-point Likert-type scale. Scores ranged from 4 to 28, with higher scores indicating increased MW tendency for each type. The Japanese versions of the scale were used 33  Analyses. This study primarily used correlational analyses. The 95% confidence intervals (CIs) for each correlation coefficient were listed. Although correlational analyses were conducted in a sequential manner, multiplicity was considered by using the Bonferroni correction given the number of possible combinations to strengthen the reliability of the results. To investigate the specificity of these associations, Fisher's r to z transformation was used to statistically compare the correlation coefficients. Although skewness and kurtosis were within acceptable ranges (< |2|) 34 (Tables 1 and 2), MW-D and MW-S seemingly intruded on the distribution of normality with a floor effect (Figs. S1 and S2). Therefore, supplemental non-parametric analyses were implemented for the correlations that included the two variables. Moreover, based on the results of the correlation, several mediation analyses were conducted to elucidate the structure of the current data. To investigate whether the sub-components (executive for motivation and spontaneous for MW) mediate the association between motivation and MW in general, the study set the independent and dependent variables as AS and MWQ, respectively. In Models 1 and 2, the mediators were MW-S and dasEx, respectively. In Model 3, MW-S and dasEx were simultaneously included. Besides the conventional z-test, bootstrapping (2,000 samples) was used to test the significance of the indirect effects, which does not require assuming normality of the sample distribution 35 . These non-nested models were compared via Vuong's likelihood ratio tests 36,37 . The analyses and visualisation were performed using R (https:// www.R-proje ct. org/). Table 1 presents the descriptive information of the participants, which is visualised in Fig. S1. First, to replicate a previous report of the significant association between motivation and MW at the trait level 13 , this www.nature.com/scientificreports/ study employed correlation analysis and found that the replication was successful, where the lower the motivation, the higher the MW rates experienced at the trait level (AS-MWQ: r = 0.39 [95% CI 0.32-0.46], p < 0.001). Figure S3 presents the correlation coefficients among the variables.

Results.
To test the hypothesis that people who are unmotivated intentionally allow their minds to wander, this study assessed the correlations between AS and MW-D/S. A significant correlation was found between AS and MW-S (r = 0.25 [95% CI 0.18-0.33], p < 0.001) but not between AS and MW-D (r = 0.10 [95% CI 0.02-0.18], p = 0.248) (Fig. 1A). These correlation coefficients differed significantly (z = 4.69, p < 0.001). The second hypothesis, that is, the executive component will modulate the relationship between motivation and MW, was also tested. MWQ was significantly correlated with dasEx (r = 0.66 [95% CI 0.61-0.70], p < 0.001) but not with the other sub-components of DAS (dasEm: r = 0.00 [95% CI − 0.08 to 0.08], p = 0.988; dasIni: r = 0.09 [95% CI 0.01-0.17], p = 0.522) (Fig. 1B). This difference was also significant (zs > 12.3, ps < 0.001). As an additional analysis, non-parametric correlational analysis was conducted for measures whose distributions violated normality. This analysis did not influence the significance of the results, as shown in the Supplementary Material.
These results indicated that the executive component of motivation and/or spontaneous MW mediates the relationship between motivation and MW. Additional mediation analyses were conducted using several models. Figure 2 presents the graphical results. In Models 1 and 2, the indirect effect was significant, indicating that the two variables significantly functioned as a mediator in the association between AS and MWQ. A comparison via the robust likelihood ratio test between the two non-nested models suggested that Model 2 is superior to Model 1 (robust likelihood ratio = 644, p < 0.001). When the variables were included simultaneously, the indirect

Interim discussion
A person's MW tendency is related to several factors 1,5,14 . Intuitively, motivation towards tasks influences the rate of MW during such tasks [6][7][8]11 . A previous study also identified this association between motivation and MW at the trait level 13 . However, the underlying mechanism remains unclear. The significance of trait-level association between motivation and MW could be replicated in Study 1 using a different sample based on a previous report 13 , which indicated that the association is robust in healthy individuals. This finding is important because this relationship was not intuitive at the trait level. Next, simple correlation analyses indicated that the executive component of motivation and the spontaneous component of MW contribute to the association. The following analyses supported the possibility that these sub-components mediate the association between motivation and MW. Statistically, the executive type of motivation and spontaneous MW partially mediated this association. Their mediating role can also be observed in the model that simultaneously posits the two variables as mediators (i.e. Model 3), although Model 2 displayed superiority in terms of parsimoniousness. Importantly, the result of Model 3 provides knowledge that dasEx entirely mediates the effect of AS on MW-S, whereas MW-S significantly mediates the relationship between dasEx and MWQ. This outline is reminiscent of the executive failure hypothesis of MW 3,15 . As this hypothesis is in accordance with the traditional perspective: MW is a result of a lapse in attention; participants with low levels of executive function would experience high levels of MW than those with high levels of executive function. Theoretically, this model focuses on spontaneous MW, which should be experienced after the failure of the executive process 10,32 . This model supports the current results because the executive type of amotivation influenced the occurrence of spontaneous MW, which could contribute to the general association between amotivation and MW at the trait level. Thus, the key psychological component of the association between motivation and MW is executive function.
However, to be precise, dasEx is not a measurement of executive function although it is conceptually clear. Study 2 provides a more direct assessment of executive function and examines its effect on the relationship between motivation and MW using ECS, which is used to assess executive function, especially for inhibition, initiation, and attention control 22,23 , in addition to confirming the reproducibility of Study 1 in another sample of Study 2.

Study 2
Participants. Basically, the recruitment used for Study 2 is similar to that of Study 1. The same data collection company was used to recruit participants for Study 2; however, they were independently recruited. This study estimated that 150 participants would be included for every age group (i.e. 20-29, 30-39 and the like), resulting in 750 participants in total. Again, three lure items were set, where 188 participants did not follow the instructions. Finally, data from 562 participants were analysed (mean age = 45.4 ± 13.7 years; women = 263). Only certain participants (n = 35) were duplicated between the two studies, identified by the ID provided by the data collection agency.
Measurements. Besides the measurements in Study 1, a new measurement was added to assess participants' executive function.
Scale for executive function. Effortful control scale (ECS). ECS is a questionnaire used to measure participants' effortful control similar to executive function 22,23 . The questionnaire comprises 35 items, including reverse items, which were rated using a 4-point Likert-type scale. Effortful control is a single, latent temperamental construct directly linked to executive function 38 , which includes sub-components of inhibition (e.g. 'It is easy for me to hold back my laughter in a situation where laughter wouldn't be appropriate' and 'If I want to, it is usually easy for me to keep a secret'), activation (e.g. 'I am often late for appointments' and ' As soon as I have decided upon a difficult plan of action, I begin to carry it out'), and attention control (e.g. 'It is very hard for me to focus my attention when I am distressed' and 'When I am trying to focus my attention, I am easily distracted'). High scores indicate better executive control functions.
Analyses. The analytical methods used in Study 1 were identical to those used in Study 2, except for the addition of another mediation model for verifying the mediating role of executive function in the relationship between motivation and spontaneous MW (i.e. independent variable = MW, mediator = ECS, and dependent variable = MW-S), which was hypothesised in Study 1. Similarly, bootstrapping (2000 samples) was used besides the conventional z-test to confirm the significance of the indirect effects, which does not require assuming normality of the sample distribution. Table 2 provides the descriptive information of the results, which are illustrated in Fig. S2. Figure S4 provides the correlation coefficients among the variables.

Results.
To replicate the association between motivation and MW at the trait level, this study employed correlation analysis and found that the replication was successful (AS-MWQ: r = 0.25 [95% CI 0.11-0.37], p < 0.001). Moreover, Study 1 results were also reproduced. As Fig. 3A shows, the selective significant associations between AS and MW-S (r = 0.19 [95% CI 0.05-0.32], p < 0.001) and between MWQ and dasEx (r = 0.60 [95% CI 0.50-0.68], p < 0.001) were confirmed. The remainder of the associations were non-significant (ps > 0.05). www.nature.com/scientificreports/ Additionally, Study 2 re-conducted the mediation analyses conducted in Study 1 to confirm the replicability of Study 1 on the independent samples. As Fig. 3B shows, analyses extended the results in Study 1 through the finding that the association between AS and MWQ was entirely explained by the variance of dasEx, which indicates the possibility of executive function. The bias-corrected bootstrapped CIs for indirect effects were above 0 in all models (0.04-0.11 in Model 1, 0.11-0.20 in Model 2 and 0.11-0.20 in Model 3).
Thus far, the study confirmed the possibility that the key component of the association between motivation and MW is executive function, which in turn causes spontaneous MW. This relationship was tested using a more direct measure of executive function (i.e. ESC). Correlation and mediation analyses revealed that ECS is more strongly associated with MW-S than MW-D (z = 7.83, p < 0.001), although these correlation coefficients were significant (ECS-MW-S: r = − 0.54 [95% CI − 0.59 to − 0.47], p < 0.001; ECS-MW-D: r = − 0.28 [95% CI − 0.35 to − 0.20], p < 0.001). Additionally, ECS mediated the relationship between AS and MW-S (Fig. 4). The biascorrected bootstrapped CIs for indirect effect ranged from 0.11 to 0.20.

Discussion
The present study demonstrated that the trait-level association between motivation and MW is primarily mediated by executive function, which then evokes spontaneous MW. These findings were reliable as per the replicability of the study using a different sample in Study 2.
Previous studies have reported that motivation and MW are temporally related 6,7,11 . A proposed mechanism of this state-level association indicates that individuals deliberately allow the mind to wander during unmotivated periods. In other words, individuals are expected to deliberately let their mind wander during unmotivated states in daily life. However, the current results did not support this hypothesis. The result is understandable given the dissociation between the trait and state dimensions in motivation and/or MW. Previous studies have also indicated that motivation and MW in fact share a covariance between the trait and state dimensions; however, these dimensions are weakly associated 9,13,39 . The trait-state relationship is enigmatic and is influenced by several factors, such as memory, metacognitive ability and time or season of the test. The current study provided an insight into understanding the divergence between the trait and state dimensions. Contrary to the hypothesis that amotivation induces deliberate MW, this study found that the lower the level of motivation, the more the mind wanders in a spontaneous fashion at the trait level, whose association depends on executive function.
The concept of the subdivision of motivation in the current study emerged from the perspective of apathy 17,18,40 . Although the importance of the subdivision in healthy individuals remains unclear, the executive component may be the most important factor of apathy in patients without dementia 41,42 and healthy adults 19,20 . In this study, the subdivision suggested that executive dysfunction may be the mechanism that underlies the association between motivation and MW, which could lead to spontaneous MW at the trait level. Using a more direct assessment of executive function can corroborate this finding. As noted in the Interim Discussion section, the executive failure hypothesis of MW supported the results. Individuals with high levels of executive capacity reported more on-task and less off-task thoughts during experimental tasks compared to those with low levels of executive capacity 3,6,14,15 . Based on such a clear relationship, this hypothesis suggests that the inability to focus on a task and task-relevant goals could result in MW. Specifically, the experience sampling method found that participants with low levels of executive function experienced high rates of MW in daily life in terms of the effect of executive function on MW at the trait level 43 . This finding is consistent with the current results. Conversely, several studies have proposed the link between high levels of cognitive capacity and increased MW 4,44 . This seemingly contrasting phenomenon, explained by executive failure and executive control accounts, are perhaps dependent on a certain context, that is, the demands of the task 5,45 (see study 46 for another reconciliation). People with high levels of executive skills could adaptively select their mental activities from avoiding MW that leads to deleterious consequences during demanding tasks 6,15 to allowing their mind to wander during non-demanding tasks 4,14,44 . In the latter case, studies have confirmed the adaptive aspect because people could consider the future 47 or think in a creative manner 48 . Based on this account of context regulation hypothesis 45 , the current study suggests that the introspective nature of a questionnaire survey weights or biases MW during specific tasks in daily life with relatively high demands. As an interpretation, people tend to remember these MW more clearly than the MW during tedious tasks or no task because such MW during tasks in high demands would result in negative consequences. Perhaps this introspective and/or memory bias could explain the absence of correlation between deliberate MW and trait motivation, although there are other possibilities (e.g. people with less motivation at the trait level may lack the motivation to let their mind wander deliberately because deliberate MW needs a cognitive load to a certain extent). Because the specific measures used here cannot support or deny this hypothesis, future research may need to adopt measurements for the contexts that individuals are in when they are MW so as to consider the inference of an introspective bias.
The potential limitations of this study are as follows. First, the causality included in the mediation analyses seems inappropriate because the causal effect of amotivation on executive function may be theoretically questionable, especially for the model shown in Fig. 4. The result should be interpreted as follows: the covariance between executive function and amotivation evokes spontaneous MW and diverts attention from the causal relationship. Second, the application of the apathy subdivision to motivation in healthy individuals may be controversial. Although apathy is clearly a disorder of motivation 16 several apathy scales had been initially developed by employing healthy populations 18,29,40 , one can claim that this application is inadequate. Moreover, the reliability of dasEm was insufficient in both Studies 1 and 2, which was also demonstrated previously 29 . Further investigation of these points may be required. Third, the retrospective method may bias the results. Besides the aforementioned nuisance effect of memory, such self-reported data would be contaminated with the mood and neurocognitive state during the survey 5 ; moreover, trait-state and survey test inconsistency were observed in MW 13,49 . Moreover, studies on executive function have demonstrated such a discrepancy 50 . To draw solid conclusions, an experimental study that utilises task-based assessments for MW (i.e. experience sampling method) and executive function (i.e. Stroop's task and N-back task) are warranted. Fourth, the characteristics and conditions of the participants were totally unknown, except for age and sex. The study did not conduct screening for cognitive impairment.
Irrespective of these limitations, the current findings can provide meaningful insights for understanding motivation and MW at the trait level and aid in developing interventions to control these tendencies in daily life. Although this study used the correlational approach, its adequate sample size and self-replication provided a solid conclusion that the failure of executive function can explain the association between motivation and MW at the trait level, which may be biased by the use of retrospective methods.
Ethical approval. All procedures performed in the studies involving human participants were conducted in accordance with the ethical standards of the Institutional Review Board with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent was obtained from all participants.